Head-to-tail macrocyclization of peptides and proteins has been used as a strategy to constrain structures and enhance metabolic stability against proteolytic degradation. In addition, a constrained macrocyclic conformation may also improve pharmacological activity and oral bioavailability. Although most peptides and proteins are produced as linear chains, circular peptides ranging from 6 to 78 residues occur naturally in diverse organisms. These cyclic peptides usually display high resistance to heat denaturation and proteolysis and have inspired a new trend in protein engineering, as demonstrated by recent successes in the cyclization of cytokines, histatin, ubiquitin C-terminal hydrolase, conotoxin and bradykinin-grafted cyclotides. Furthermore, cyclic peptides have been used as therapeutics, including valinomycin, gramicidin S and cyclosporine.
To date chemical methods are typically used for the cyclization of peptides. One possible strategy is native chemical ligation. This method requires an N-terminal cysteine and a C-terminal thioester, requirements that limit its application for non-cysteine-containing peptides. Furthermore, chemical methods are not always feasible, especially for large peptides and proteins.
Although enzymatic methods employing a naturally-occurring cyclase would be ideal, currently only very few peptide cyclases are known and they are for various reasons not fully exploited. However, other enzymes such as sortase A and inteins of which the innate functions are not a cyclase have been applied successfully for cyclization of various peptides and proteins. Nonetheless, these enzymes have shortcomings. Sortase A, for example, is a transpeptidase that anchors surface proteins to bacterial cell wall. Its cyclization reaction usually requires an overnight incubation and 0.1 to 1 molar equivalents of enzyme. Furthermore, sortase A has a pentapeptide recognition sequence LPXTG (SEQ ID NO: 160) and leaves an unnecessary tag on the modified proteins. Inteins are autocatalytic splicing elements that have been used for expression of cyclotides, sunflower trypsin inhibitor, and q-defensin. The intein-mediated cyclization, however, requires genetic fusion of a target protein with the intein domain, a necessity that may affect the protein folding or solubility.
There is thus still need in the art for improved means to cyclize peptides and proteins that overcome the drawbacks of existing technologies and, ideally, are simple, fast and versatile.